Many current automotive vehicles are provided with visual displays for presenting information to the driver or passengers. Usually such displays are built into the infotainment systems or the instrument cluster. The graphical content of these displays is usually generated by a controller which also handles user inputs to the system. Generally, the controller contains at least a CPU (central processing unit) and a GPU (graphic processing unit). Often there is memory connected to or integrated with the controller. The memories can either be volatile memory or permanent storage type. US 20130016209 A1 discloses a vehicle display which can display a video image captured by an on-board camera or a map downloaded from a remote service centre for navigational purposes or written instructions for following a particular route to a desired destination. US 20130050271 A1 discloses a vehicle display apparatus which displays the same information displayed by a screen of a portable terminal, for example, a mobile phone, while the portable terminal is temporarily coupled to in-vehicle apparatus. Both the portable terminal's display screen and the vehicle's display screen may be touch sensitive panels or “touchscreen” panels. Touchscreen panels are well known, one of the most common types being the capacitive touchscreen comprising an insulator (for example, glass) coated with a transparent conductor. Touching of the screen's surface by a human finger (which is slightly electrically conducting) results in a distortion of the screen's electrostatic field, measurable as a change in capacitance. Various technologies may be employed to determine the location of the fingers contact point. Some touchscreens may also react to movement of the finger across of the screen so that, for example an image of a map may be moved across the screen to the left or right or up and down. US 20120327106 A1 discloses a touchscreen display which is partitioned into two regions. The same image (or “graphic content”) may be displayed on each region but with a different orientation. For example one may be the mirror image of the other. A general problem with graphic content from different sources is to associate the graphic content with the source which provided it. In the case of the touchscreen, the touch position refers to a specific graphic content displayed on the screen. Such graphic content may be for example a representation of a button (a soft key). The user interaction usually has to be handled by the system which generated this content. Therefore, an association of user inputs to the source of the graphic content has to be provided. While this is a rather trivial task when the different sources allocate clear rectangular areas on the screen, in the case where images from sources are merged, determining the source of the graphic content at any particular location on the screen is no longer so trivial. One known method for merging images from two different sources employs the concept of colour keying. This is a technique which is often used in the broadcasting industry for superimposing one video image onto another. A colour or colour range of an overlaid image is made transparent revealing another image behind it. Usually a (monochrome) background of the overlaid image is “keyed out” so that the subject of the overlaid image may be superimposed onto a different background. US 20010014175 A1 describes such a method.